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The photoperiodic flowering pathway is one of the most important regulatory networks controlling flowering time in rice ( Oryza sativa L.). Rice is a facultative short-day (SD) plant; flowering is promoted under inductive SD conditions and delayed under non-inductive long-day (LD) conditions. In rice, flowering inhibitor genes play an important role in maintaining the trade-off between reproduction and yield. In this study, we identified a novel floral inhibitor, OsCOL15 , which encodes a CONSTANS-like transcription factor. Consistent with a function in transcriptional regulation, OsCOL15 localized to the nucleus. Moreover, OsCOL15 had transcriptional activation activity, and the central region of the protein between the B-box and CCT domains was required for this activity. We determined that OsCOL15 is most highly expressed in young organs and exhibits a diurnal expression pattern typical of other floral regulators. Overexpression of OsCOL15 resulted in a delayed flowering phenotype under both SD and LD conditions. Real-time quantitative RT-PCR analysis of flowering regulator gene expression suggested that OsCOL15 suppresses flowering by up-regulating the flowering repressor Grain number , plant height and heading date 7 ( Ghd7 ) and down-regulating the flowering activator Rice Indeterminate 1 ( RID1 ), thus leading to the down-regulation of the flowering activators Early heading date 1 , Heading date 3a , and RICE FLOWERING LOCUS T1 . These results demonstrate that OsCOL15 is an important floral regulator acting upstream of Ghd7 and RID1 in the rice photoperiodic flowering-time regulatory network. [ABSTRACT FROM AUTHOR]